In a product assembly plant, such as an automobile assembly plant or an electrical appliance assembly plant, when parts are transported from a parts manufacturer's plant into the product assembly plant, when the parts transported into the assembly plant are transported to an assembly site, or when the parts are transported from the assembly site to another assembly site, usually, the parts are placed in containers and the containers are manually transported by workers. It is desired to reduce as much as possible the gross weight of a returnable container that can be transported by a worker from the standpoint of preventing the worker from suffering lower back pain and injuries. According to the National Institute for Occupational Safety & Health (NIOSH), it is recommended that the maximum gross weight of a returnable container be 51 pounds (23 kg) or less. Actually, most plants have their own rules, and in many cases, the maximum gross weight is set at 25 to 50 pounds (11.33 to 22.68 kg).
Meanwhile, returnable containers are required to have sufficient strength for carrying parts. Consequently, as such returnable containers used for transporting parts by workers in product assembly plants, containers formed by injection molding or press molding of a polyolefin resin or the like are conventionally used. In some cases, the returnable containers may be reinforced with metal fittings in order to improve strength as required. The returnable containers have a density of 900 to 1,200 g/L, depending on the resin or formulation used, and the returnable containers themselves are heavy. Consequently, the percentage of the weight of a returnable container relative to the gross weight of the returnable container that can be transported by a worker is large.
U.S. Pat. No. 3,508,679 discloses a returnable container for transporting parts from one location to another on a conveyor in a plant, the returnable container being provided with bumper elements to prevent damage which might result from succeeding returnable containers bumping into each other on the conveyor.
Japanese Unexamined Patent Application Publication Nos. 2002-128072, 2007-62764, 2005-206210, etc. disclose foamed synthetic resin containers, which are mainly used as containers for transporting seafood, vegetables, etc. As the resin used for the containers, for example, in addition to polystyrene, olefin-based resins, such as polyethylene and polypropylene, are described therein.